Loran-C and other radio-wave pulse train navigation systems have been designed to serve also to transmit and receive digital communication messages generally by pulse-position modulation (PPM) involving appropriate encoding of communication messages and logical multiplication and inversion of encoded signals prior to phase modulation of the Loran-C pulses, as described, for example, in U.S. Pat. Nos. 4,800,391 and 4,821,038 of Megapulse, Inc., the common assignee of the present application. With Loran-C 100 K Hz carrier frequency systems, such modulation enables a digital bit rate of 70 bps.
A current example of such PPM is the Eurofix System wherein a seven-bit word is created using the last six pulses of an eight-pulse group. To enhance the Loran data bit rate capability, schemes such as increasing the number of pulses from 300 pps to 500 pps (sometimes referred to as Supernumerary Loran) and using a three-level Intrapulse Frequency Modulation (IFM) on such system pulses are being discussed.
It would, however, be highly desirable, particularly with existing Loran-C transmitter stations worldwide, if the bit rate could indeed somehow be increased to 250 bps and higher with the current station equipment, and still without affecting the integrity of the navigation capability of the Loran-C system.
The present invention provides such a break-through: the discovery of a novel modulation scheme readily implementable in current (and future) Loran-C (and similar) transmitters, wherein a novel type of frequency modulation is added on the Loran-C transmitted pulses.
The primary object of the invention, accordingly, is to provide a new and improved method of and apparatus for expanding the digital bit-rate potential for message communication to be supplementarily carried on Loran-C radio navigation transmissions and the like, without, however, affecting the principal radio navigation capabilities thereof.
A further object is to accomplish such an end with electronic circuitry that lends itself to ready modification of the current worldwide Loran-C (and similar) transmitter equipments.
Other and further objects will be explained hereinafter and are delineated, also, in the appended claims.
In summary, from one of its important viewpoints, the invention embraces a method of expanding the digital bit rate potential for communication to be added to Loran-C and similar radio-frequency navigation system transmitter pulse trains and without affecting the navigation capability and integrity of the system, that comprises, generating such radio-frequency pulses centered on a predetermined carrier frequency and transmitting the same from antennas to enable users to receive and use the initial part of each transmitted pulse for navigation timing and location; during the remaining parts of each transmitted pulse, sweeping the generated carrier frequency thereof in opposite directions, first below or above, and then above or below the carrier frequency during successive further parts of each pulse, but within a predetermined band between predetermined lower and upper frequency limits on opposite sides of said carrier frequency; further, during said sweeping, maintaining the pulse energies below the lower frequency limit and above the upper frequency limit substantially equal for each pulse: and using the frequency modulation effected by said sweeping to provide communication bits.
Preferred and best mode implementations and designs for practicing the invention are hereinafter explained in detail.